In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a silicon substrate or the like, a fine pattern, which is required for writing an integrated circuit comprising such a fine pattern. However, the conventional exposure techniques using light exposure have been close to the limit of the conventional photolithography method while semiconductor devices have had finer patterns at an accelerated pace. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, has been considered as being promising as the exposure technique for 45 nm or below. In this Description, it should be noted that the phrase “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
It is impossible to use EUV light in conventional dioptric systems as in photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since the refractive index of the substances is close to 1 at the above wavelength. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member to be used for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, formed thereon in this order. The reflective layer normally comprises a reflective multilayer film, which comprises molybdenum (Mo) layers as high refractive layers and silicon (Si) layers as low refractive layers alternately stacked to increase a light reflectance when irradiating a layer surface with EUV light.
The absorber layer comprises a material having a high absorption coefficient to EUV light, specifically, for example, a material containing chromium (Cr) or tantalum (Ta) as the main component.
Between the reflective layer and the absorber layer, usually a protective layer is formed to prevent the surface of the reflective layer from being oxidized. As a material of the protective layer, Si has been widely used. On the other hand, Patent Document 1 proposed use of material (Ru) as the material of the protective layer.
In a case where Ru is used as the material of the protective layer, a high etching selectivity of the absorber layer will be obtained and further, a high reflectance will be obtained as compared with a case where a Si film is used as the protective layer. However, in a case where Ru is used as the material of the protective layer, a diffusion layer will be formed between the Ru protective layer and the outermost Si layer of the reflective multilayer film in a heating step (e.g. resist baking step) in production of a mask or during EUV exposure, and further, the diffusion layer will be oxidized, thus leading to a decrease in the reflectance of the reflective layer.
Particularly, since the decrease in the reflectance during EUV exposure proceeds with time, the exposure conditions should be changed during operation, or the life of the mask may be shortened.
In order to solve such problems, Patent Document 2 proposes a protective layer made of a ruthenium compound (Ru content of from 10 to 95 at %) containing Ru and at least one member selected from Mo, Nb, Zr, Y, B, Ti and La.
It is disclosed that by using such a protective layer, formation of a diffusion layer between the protective layer made of the ruthenium compound and a Si layer as the outermost layer of the reflective multilayer film in a heating step (resist baking) in production of a mask can be suppressed.
Patent Document 1: JP-A-2002-122981
Patent Document 2: JP-A-2005-268750